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A simple matter of complexity: Does life have to become more

By ROGER LEWIN

‘The course of evolution is uncertain, but its patterns are not,’ observes Roger Thomas, a palaeontologist from Franklin and Marshall College, Pennsylvania. ‘The most significant pattern in the history of life is the progressive net increase in complexity of structure and dynamics that has occurred in organisms and the ecosystems in which they participate.’ With this simple, straightforward statement, Thomas plunges headlong into one of the more contentious issues in biology&colon; the shape of the history of life on Earth. Does evolution inevitably generate ever more complex organismsand ecosystems as time passes? And if so, what is the nature of that increase?

Even to the casual observer, the answers to those questions seem obvious. Life started with single-celled organisms, moved on to primitive multicellular organisms such as bizarre jawless fish and lumbering reptiles, and has culminated (so far) in hot-blooded, sleek and fleet-of-foot mammals, some of which are also endowed with large brains and even consciousness. Intuition tells us that mammals are somehow more complex than reptiles and that reptiles are more complex than invertebrates. Evolution seems to move in the direction of ever increasing complexity. But is it possible to devise experiments to test this assumption?

So far, few researchers have tried. The difficulty is not in recognising complexity, but in measuring it. Comparing a cat with a clam, many people will feel that there is ‘something more’ going on in the cat, says Dan McShea, a palaeontologist at the University of Michigan, Ann Arbor. ‘But is that ‘something more’ greater complexity or is it greater intelligence, greater mobility, or greater similarity to us? Hard …